The present invention relates to flow measurement devices and methods, and in particular to flow measurement devices and methods using ultrasonic energy to determine flow rate.
Flow meters using ultrasonic transducers, particularly non-intrusive transducers of the clamp-on variety, are known. For example, ControLotron Corporation of Hauppauge, N.Y. produces flow meters which have transducers that clamp-on to pipes of pipelines for non-intrusively determining the flow rate within the pipeline. According to these types of flow meters, ultrasonic transducers are clamped onto the pipe wall, and an ultrasonic signal is transmitted into the pipeline wall and emerges into the fluid and flows through the fluid traversing in the pipeline. The difference between the upstream and downstream transit times of sonic energy transmitted diagonally through the fluid in the pipeline is then used to determine the flow velocity according to well-known principles. See, for example, U.S. Pat. Nos. 4,232,548 and 5,453,944. In particular, the flow velocity is determined by the following formula: VF=K.DELTA.t/TL where VF equals the flow velocity, K equals a dimensioned calibration factor in units of volume/time, .DELTA.t equals the measured upstream minus downstream transit-time difference and TL is the measured average upstream and downstream transit time.
Such transducers may be of the wide beam path type as disclosed in U.S. Pat. No. 3,987,674. Wide beam transducers are matched to the resonant frequency and phase velocity of the pipe by suitable selection of the angle at which the transducer is mounted with respect to the axis of the pipe and selection of the material of the transducer body. Basically, it is necessary to choose the angle and a material for the transducer body which has a longitudinal mode sonic velocity that is less than the shear mode velocity of the pipe or conduit material. This is necessary so that the phase velocity of the sonic energy in the transducer housing can be adjusted to match the shear mode velocity of the pipe.
As well known in the art, the transducers may be arranged on opposite portions of the pipeline wall or they may be arranged on the same side of the pipeline by utilizing the reflection from the opposite wall portion.
Also known is that when fluid flows through a pipe, the Reynolds number N.sub.R affects the profile of the fluid through the pipe. By profile is meant the profile of the velocity vector across the cross sectional area of the pipe. It is known that in steady state conditions, the velocity at the center is generally higher than the velocity near the walls of the pipe. Also known is that the higher the Reynolds number, the flatter the flow profile. Such factors as the diameter of the pipe, the viscosity and flow rate all factor into the determination of the Reynolds number N.sub.R.
The Reynolds number also determines the transition from what is known as laminar to turbulent flow. Typically, the transition occurs for Reynolds numbers between 2000 and 4000.
A problem with the steady state flow profile is that the determination of the flow volumetric rate is not independent of the Reynolds number. Unless Gaussian quadrature chordal summation techniques, known to those of skill in the art, are utilized, it is difficult to obtain the correct flow rate because the flow rate is dependent on the Reynolds number. In order to determine the flow rate accurately, it is useful to apply the Gaussian quadrature chordal summation technique because this technique makes determination of flow rate relatively independent of Reynolds number.
The Gaussian quadrature chordal summation technique is typically used to determine flow rate when the fluid flow exhibits a non-flat flow profile. When there is a non-flat flow profile this technique can be used to determine the true flow rate based on the outputs from ultrasonic flow measurement transducers. It is not needed when the flow profile is flat. In order to determine the flow rate accurately, it is preferred that the entire flow within the pipe should be illuminated by the ultrasonic energy. If more than one transducer is used to illuminate the flow, the results from the plurality of transducers must be appropriately summed. Assuming that the ultrasonic transducers utilized to determine the flow rate fully illuminate the flow profile within the pipe, it is possible using Gaussian quadrature chordal summation techniques to determine the correct flow rate when there is a non-flat profile. This summation technique is required because to establish chordal illumination in a round pipe requires inserting transducers into the flow stream, resulting in unequal chord lengths and unequal sensed volumes. If the flow profile is almost flat, the Gaussian quadrature chordal summation is less necessary, but if applied, will compensate for the shape of the profile. Alternatively, flow rate can be obtained by simple averaging over a number of parallel paths illuminated by the transducers, but this is possible only for paths of equal length and volume.
The use of the Gaussian quadrature chordal summation technique is somewhat complex requiring individual processing of each path's data and it would be advantageous to be able to determine flow rate accurately without resorting to this technique.
Another problem in prior art ultrasonic flow meters is that unwanted pipe "noise" signals are received at the receive transducer which have not traversed the liquid in the pipe but instead have travelled through the pipe wall to the receive transducer. Although some of these unwanted pipe noise signals can be eliminated, if they arrive at a different time from the liquid signal or are asynchronous, some can not because they may occur about the same time as the receive signal through the liquid in the pipe, thus leading to corruption of the desired receive signal and possibly incorrect timing determination and consequently incorrect flow rate.